This invention relates to hydraulic tensioners used to apply proper tension to a timing chain, belt, or the like in a vehicle engine, and more particularly to a tensioner having a relief valve mechanism.
These tensioners are typically installed in the manner depicted in FIG. 9, where a hydraulic tensioner 50 is shown in conjunction with a timing transmission in FIG. 9. The tensioner 50 adjusts chain tension through a tension lever TL, and a relief valve (not shown in FIG. 9) is provided in the tensioner to relieve excess tension generated in the timing chain TC, when the timing chain is stretched as it transmits power.
The relief valve mechanism is shown in a cross-sectional view of the tensioner in FIG. 10. The tensioner comprises a housing, or tensioner body, 51, and a hollow plunger 52, slidably fitting into a blind plunger-accommodating hole 51a formed in the housing 51. An end wall 52a of the plunger 52 is outside the housing. A spring 53, inside the plunger 52, biases the plunger in the projecting direction. A high pressure oil chamber 54 is constituted by the interior of the plunger and a part of the blind hole 51a behind the plunger. A check valve mechanism 55 is provided at the bottom of the plunger-accommodating hole 51a, and a relief valve mechanism 57 is provided in the housing 51 at a location such that it communicates with the oil chamber 54 through a passage 57b, which leads to an opening in the side wall of the plunger-accommodating hole 51a, near the bottom of the hole.
The check valve mechanism 55 comprises a ball seat 55b having an oil passage 55a. The seat 55b is press-fit into a hole 51b formed at a bottom portion of the plunger-accommodating hole 51a. A check ball 55c, which can abut the ball seat 55b, is held in close proximity to the seat by a retainer 55d. A spring 55e, shown schematically, biases the check ball 55c toward the ball seat 55b. An oil supply passage (not shown), for supplying oil from a source of oil under pressure to the high pressure oil chamber 54, communicates with hole 51b in the housing 51. The check valve mechanism 55 permits the flow of oil into the high-pressure oil chamber 54, but blocks flow out of the high pressure oil chamber 54 in the reverse direction.
The relief valve mechanism 57 comprises a valve element fitting recess 57a formed in the housing 51, the oil passage 57b, which provides fluid communication between the high pressure oil chamber 54 and the valve element fitting recess 57a, a cup-shaped valve element 57c slidably fitting into the recess 57a with the bottom surface of the valve element 57c facing the oil passage 57b, a spring 57d, which biases the valve element 57c toward the oil passage 57b, a relief hole 57e, provided near the oil passage 57b in a side wall of the valve element fitting recess 57a, and a plug 57f press-fit into the recess 57a from the outside. The relief hole 57e provides fluid communication between the inside of the valve element fitting recess 57a and the outside of the housing 51.
When the oil pressure in the high pressure oil chamber 54 exceeds a predetermined pressure, the valve element 57c is moved backward against the biasing force of the spring 57d, and the oil passage 57b communicates with the relief hole 57e, thereby reducing the oil pressure in the high pressure chamber 54. The inside of the valve element fitting recess 57a is filled with oil to ensure smooth operation of the valve element 57c, and the oil is supplied and discharged through an oil hole 57h bored in the plug 57f as the valve element 57c moves backward and forward.
With the engine operating normally, when the timing chain loosens, the biasing force of the spring 53 in the above-described hydraulic tensioner 50 causes the plunger 52 to project. As the plunger projects, the check ball 55c separates from the seat 55b, and oil, supplied from an oil pressure source such as an oil pump or the like, flows into the oil chamber 54 through the oil supply passage (not shown). The oil supply passage communicates with the hole 51b and the oil passage 55a in the ball seat 55b, so that the oil supplied to the oil chamber 54 maintains tension in the timing chain.
When the chain is under excess tension, the plunger 52 receives a thrust load from the tensioner lever (FIG. 9) in a direction opposite to the direction in which the plunger is biased by spring 53. The check valve mechanism 55 then prevents backflow of oil from the high pressure oil chamber 54, and the oil, being non-compressible, prevents the return of the plunger 52. However, the valve element 57c of the relief valve mechanism 57 responds to the excess oil pressure in chamber 54 by sliding against the biasing force of spring 57d. A slight backward movement of the valve element 57c absorbs a small amount of excess tension in the timing chain.
When the thrust applied to the plunger 52 further increases, causing the valve element 57c to move farther in recess 57a, the oil passage 57b communicates with the relief hole 57e, permitting discharge of oil from the high pressure oil chamber 54 to the outside of the tensioner 50. Thus, the oil pressure in the high pressure oil chamber 54 decreases, and excessive tension of the timing chain is absorbed.
When the oil pressure in chamber 54 decreases as oil is discharged from the tensioner, the biasing force of the spring 57d causes the valve element 57c to close the relief hole 57e, to maintain the oil pressure in chamber 54 at a preset value. If the oil pressure in chamber 54 decreases excessively, the check valve mechanism 55 allows oil to flow into chamber 54, to return the oil pressure to a normal level.
In the conventional hydraulic tensioner 50, as shown in FIG. 10, a condition of excess tension can occur while oil continues to flow from the high pressure oil chamber to the relief valve mechanism. When this condition occurs, the valve element 57c of the relief valve mechanism 57 moves backward rapidly and collides with the plug 57f, generating hitting sounds.
The spring 57d, which biases the valve element 57c, can also become compressed past its elastic limit, with the result that the biasing force exerted by the spring 57d gradually decreases, and the relief valve ceases to function properly. To avoid these problems, it has been proposed to provide a buffer of rubber or the like between the valve element 57c and the plug 57f in the valve element fitting recess 57a. However, this has not proven to be an effective solution, since the insertion of the buffer required additional assembly manpower and increases production cost.
Accordingly, the objects of the invention are to overcome the above-mentioned problems, and to provide an inexpensive, hydraulic tensioner with a structurally simple relief valve mechanism capable of avoiding hitting sounds in the relief valve structure, and avoiding wearing out and breakage of the biasing spring in the relief valve. It is also an object of the invention to provide a hydraulic tensioner which instantaneously adjusts to rapid variations in chain tension, and operates reliably when excess oil has been supplied, when tension in the timing chain increases rapidly, and when the oil pressure inside the high pressure oil chamber has reached an excessive level.
The hydraulic tensioner in accordance with the invention comprises a tensioner housing, and a plunger for exerting a tensioning force on a flexible transmission medium. The plunger projects from a blind hole in the tensioner housing, and the blind hole and the plunger together define a high pressure oil chamber. A passage is provided for delivering oil into the high pressure chamber from a high pressure oil supply, and a relief valve is connected to the high pressure oil chamber for effecting immediate pressure reduction to avoid excessive high pressure in the chamber. The relief valve comprises a valve element receiving bore communicating with an oil supply opening, a valve element, slidable forward and backward in the valve element receiving bore, a spring, biasing the valve element toward the oil supply opening, and an oil relief hole in a peripheral wall of the valve element receiving bore, at a position where it is closed when the valve element is moved backward by a rapid increase in oil pressure at the oil supply opening, whereby a quantity of oil is trapped in the valve element receiving bore for damping of the movement of said valve element.
When the valve element moves backward to a position past its usual range of positions, as a result of a rapid increase in oil pressure from the oil supply hole, the oil relief hole is closed by the relief valve element. Trapped oil blocks further backward movement of the relief valve, damping its movement so that collision between the relief valve element and the end of the valve element receiving bore is avoided.
The oil supply opening communicates with the high-pressure oil chamber through an oil reservoir and a check valve, and preferably also through a separate path comprising an oil pressure adjusting passage.
When the oil supply opening communicates with the high-pressure oil chamber through the oil pressure adjusting passage, excessive high pressure in the high-pressure oil chamber is absorbed and relieved.
An oil supply opening in the valve element receiving bore is allowed to communicate with the high-pressure oil chamber. Thus, when the pressure in the high pressure oil chamber becomes abnormally high due to a rapid increase in the tensile force in the timing chain, the pressure is instantaneously absorbed and relieved.